This invention relates to an improvement in acoustic materials and more specifically, but not by way of limitation, to structural attenuation sandwich panels used as high speed air and gas flow surfaces of high-speed aircraft.
In the design and manufacture of sound attenuation panels having structural integrity for use in severe environments, it has been common practice to provide structure with honeycomb core material sandwiched between a perforate and imperforate sheet of material with an exposed fibrous material overlay on the outer surface of the perforated sheet. Sound attenuation panels of this type are taught by U.S. Pat. Nos. 4,242,176; 4,254,171; 4,269,882; 4,271,219; 4,291,079; 4,292,356; and 4,294,329, all of which are assigned to the assignee of the instant invention. Throughout these various patents, it is taught that a layer of adhesive isolates the perforated sheet from the porous layer overlay to prevent galvanic action therebetween when the perforated sheet and porous material are constructed of dissimilar metals. This method of isolation works well; however, it has been discovered that the thickness and the required even distribution of this layer of adhesive, even under strict quality control, is not always predictable and in some instances, the completed panel lacks complete isolation between the perforated sheet and porous material. This generation of unsatisfactory material increases the economic cost of the material. It has also been discovered in some instances that the adhesive layer isolation in some of the initially satisfactory appearing panel material has broken down after a period of use thereby requiring "on site" replacement of the failed panels. On site replacement requires that the panel be removed from the aircraft and a new panel installed. This effort is quite costly both in terms of aircraft down time and economic costs.
There has not been a completely satisfactory resolution to the initial and use isolation breakdown in this type of material until the emergence of the instant invention.